Photolithography is a process commonly used in semiconductor fabrication for selectively removing portions of a film from or depositing portions of a film onto a semiconductor wafer. A typical photolithography process can include spin coating a light-sensitive material (commonly referred to as a “photoresist”) onto the surface of the semiconductor wafer. The semiconductor wafer is then exposed to a pattern of light that chemically modifies a portion of the photoresist incident to the light. The process further includes removing one of the incident portion or the non-incident portion from the surface of the semiconductor wafer with a chemical solution (e.g., a “developer”) to form a pattern of openings in the photoresist on the wafer.
The size of individual components in semiconductor devices is constantly decreasing in the semiconductor industry. To accommodate the ever smaller components, semiconductor manufacturers and photolithography tool providers have produced higher numerical aperture (NA) photolithography systems using smaller wavelengths (e.g., 193 nm). The high NA has improved the resolution of the photolithography systems, but this enhancement in resolution comes at the expense of the overall focus budget. As a result, the focus and/or exposure control must be very precise to avoid reducing product yields. Therefore, the focus of photolithography systems must be calibrated accurately and efficiently.